Resonators are commonly used in electronics for providing a signal with accurate and stable frequency. The resonators are mostly made using quartz crystals, which have a good accuracy and temperature stability of frequency. However, the production process for producing crystal resonators is different from the process of producing most other electrical circuits, which are mainly produced of silicon. Therefore, the quartz crystal resonators are usually separate components, whereby separate phases are required in the production process of electronic devices.
The quartz crystal components also tend to be large in size. It would be desirable to provide MEMS resonators made of silicon or other semiconductor materials in order to facilitate integration with e.g. silicon based components.
One problem associated with silicon based resonators is that they have a high temperature drift of the resonance frequency. The drift is mainly due to the temperature dependence of the Young modulus of silicon, which causes a temperature coefficient of frequency (TCF) approx. −30 ppm/C. This causes the resonance frequency to fluctuate due to changes in ambient temperature. It is possible to compensate the temperature dependence with a temperature sensor and related electronic control circuitry, but it has not been possible to provide a resonator with sufficiently low temperature drift with low cost technology which would be suitable for mass production applications and would compete with quartz quality. Also, the use of a temperature compensation circuit increases the consumption of energy, which is a significant disadvantage especially in battery operated devices. Further, the compensation circuit tends to increase electric noise in the resonator circuit. It is also possible to stabilize the temperature of the resonator with temperature isolation and active compensation, for example, controlled warming/cooling of the resonator. However, this solution also increases the energy consumption and noise of the device, and makes the device complicated to produce. The temperature compensation circuits are also slow in controlling, and cannot therefore compensate fast or large changes in ambient temperature sufficiently well. On the other hand, addition of amorphous SiO2 exhibiting opposite sign of temperature drift to the structure, as used in some prior art solutions, leads to a more complex fabrication process and resonator performance trade-off.
TCF in a silicon resonator in in-plane resonance modes has been recently studied e.g. in Lin, A. T.-H. et al, “Electrostatically transduced face-shear mode silicon MEMS microresonator”, Frequency Control Symposium (FCS), 2010 IEEE International, Jun. 1-4, 2010, pp. 534-538, orally held on Jun. 6, 2010, published on Aug. 23, 2010.
Another problem associated with silicon based resonators relates to their actuation. In electrostatic actuation electrodes are placed on one or more sides of the resonator body in such a way that a narrow gap is formed in between the resonator body and the electrode. A voltage between the resonator and electrodes results in an electrostatic force, which can be used for driving square-extensional or Lamé resonance. For example, Mattila et al, “Silicon Micromechanical Resonators for RF-Applications”, Physica Scripta. Vol. T114, 181-183, 2004, show an electrostatically actuated silicon resonator exhibiting square-extensional mode. However, to obtain strong enough electromechanical coupling, electrostatic actuation requires in general large (>20 V) bias voltages and narrow (<200 nm) gaps between a transducer element and the resonator. The bias and gap constraints are considerable disadvantages as concerns IC design and MEMS processing. In particular, the bias constraint is a complication for oscillator drive IC design, as low-cost processes are not compatible with voltages less than ˜5V and the on-IC DC voltage generation is power consuming. The gap constraint is a process complication, because typical commercial MEMS processes are only capable of >=2 μm gaps. Narrow gaps are also an ESD risk, reducing device reliability.
J. S. Wang et al, “Sputtered C-Axis Inclined Piezoelectric Films and Shear Wave Resonators”, Presented at the 37th Frequency Control Symp., Philadelphia, 1-3 Jun. 1983, 1983, 1-3, present that the p+-doped silicon appears to have positive temperature coefficient and in combination with ZnO and AlN films such silicon can be used for manufacturing inclined-angle shear mode resonators with an overall temperature coefficient near zero.
Lately it has been shown by A. K. Samarao et al, “Passive TCF Compensation in High Q Silicon Micromechanical Resonators,” IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2010), 2010, pp. 116-119, that heavy p-doping of silicon dramatically reduces the TCF of a Lamé resonance mode of a square plate resonator. This kind of compensation is also discussed by A. K. Samarao et al, Temperature compensation of silicon micromechanical resonators via degenerate doping, Electron Devices Meeting (IEDM), Dec. 7-9, 2009, IEEE International, IEEE, Piscataway, N.J., USA, pp. 1-4, published on Mar. 29, 2010.
US 2010/0127596 discloses a MEMS resonator which includes a boron-doped resonator region in order to reduce the TCF. The resonator may comprise a piezoelectric layer on top of the resonator and input/output electrodes on top of the piezoelectric layer. An alternative structure is disclosed in U.S. Pat. No. 4,719,383.
Beam resonators are also known which oscillate in a torsional mode. Such resonators are known, for example from T. Corman et al., “Gas damping of electrostatically excited resonators,” Sensors and Actuators A: Physical 61, no. 1 (1997): 249-255. Such resonators can also be temperature compensated. However, it is not known how such resonators could be excited using a piezoelectric thin film. Due to certain design constraints associated with electrostatic excitation, piezoelectric excitation would, however, be a preferred option in many cases.
In addition to the specific disadvantages referred to above, the above-mentioned structures are complicated to manufacture or their temperature compensation is not at a desired level. Additionally, they are generally incapable of operating at low enough frequencies (down to 30 kHz). In particular, despite recent developments in passive temperature compensation by doping of silicon, new resonator designs which can better take advantage of the capabilities of doped silicon are needed.